Tuning arrangement



Patented Nov. 12, 1946 TUNING ARRANGEMENT Arthur L. Samuel, Summit, N.3., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Original application May 6, 1942, SerialNo.

Divided and this application October 19, 1942, Serial No. 462,537

1 Claim.

This invention relates to electronic devices, particularly for operationat ultra-high frequencies.

An object of the invention is to facilitate the operation and tuning ofamplifiers, oscillators and the like, especially those employingresonating chambers or cavity resonators.

A feature of the invention is a provision for a precise or fineadjustment of the tuning of a cavity resonator comprising an adjustablebranch chamber of such restricted dimensions compared with the maincavity that the resonant frequency of the main cavity is close to andsomewhat below the range of free transmission of electromagnetic wavesin the branch cavity so that the latter will not fully support a wave ofthe frequency to be resonated and adjustment of the size or shape of thebranch cavity will have a definite but relatively small effect upon theresonance of the arrangement as a whole.

Another feature of the invention is the provision of a plurality ofloosely coupled branch -cav ities adjustable to resonate at the samefrequency as the main cavity or at slightly di-iferent frequencies,producing in either case a broad frequency band or relatively flattransmission characteristic for the system.

This application includes subject-matter originally disclosed in mycopending application, Serial No. 412,067, filed September 24, 1941, andis a division of my copending application, Serial No. 441,937, filed May6, 1942.

Further objects and features of the invention will be apparent from thefollowing detailed description and the accompanying drawing, while thescope of the invention is defined in the appended claim.

The single figure of the drawing represents an embodiment of theinvention in a frequency conversion system employing resonating chambersor cavity resonators.

Referring to the drawing, an evacuated, insulating envelope I isrepresented as enclosing a plurality of elements including a suitablesource of an electron stream such as an electron gun or beam projectorshown generally at 2,-and an electron intercepting elctrode or collector3. The electron gun 2 is provided with an electron emitting cathode 9which may be associated with any suitable heating means energized, inthe embodiment illustrated, through leads I and II by a source I2 ofelectromotive force. Associated with the cathode '9 there may be anelectrode I3 for use in regulating and varying the current of theelectron beam and commonly known as an accelerating electrode. It may beadjacent to and coaxial with the cathode.

A plurality of conductive discs H I to 2I'I, in. elusive, arehermetically sealed into and through the envelope I in any known manner.The discs are spaced at suitable intervals along the path of 2 theelectron stream as hereinafter described, and each is provided with acentral aperture in alignment with the axis of the electron gun 2 inorder to accommodate the passage of the electron stream from said gun tothe collector 3. A pair of adjacent discs 2 and M2, respectively, formparts of opposing walls of ,an input resonator I20. The disc 2II mayhave attached thereto a short tube 2 I 8 aligned with the aperture andextending outwardly with respect to'the walls of the resonator I20. Thedisc 2! 2 has fastened to the edge of the aperture therein a relativelylong, flaring tube I22. Longitudinally of "the path of the electronstream a gap I2I is defined between the central portion of the innersurface of the disc 2H and the small end of the tube I22. The disc 2I3has fastened to the edge of the aperture therein a ring-shaped electrodeI 24 positioned adjacent to the large end of the tube I22. An annulargap I23 is defined between the adjacent ends of the tube I22 and theelectrode I22. A pair of ad jacent discs 2M and 2-I5, respectively, formparts of opposing walls of anintermediate resonator I 23. The disc 2Msupports a flaring tube I26 similar to but inverted with respect to thetube I22 with the large 'end of the tube I26 defining together with theupper edge of the electrode I24 an annular gap I25. The disc 2-I5 mayhave, attached to its outer side and surrounding the aperture, a shorttube 2H. A gap I2'I is defined between the central portoin of the innersurface of the disc 2!?) and the small end of the tube i2 6.

The walls of the resonator I22 should be conductive, or should at leasthave conductive inner surfaces. The tubes I22, I25, 2I8 and 2I9 and thering I24 should be conductive and should be conductively fastened totheir respective supporting discs.

The portion of the arrangement so far described is similar to onedisclosed and claimed in my copending application, Serial No. 388,031,filed April 11, 1941, and assigned to the assignee of the presentapplication. The resonator I20 may be connected with a suitable waveguide for supplying incoming waves to the resonant system, the guidebeing separated from the resonant chamber by a conductive partition I6I. Coupling between the guide and the resonant chamber may be effectedby means of a suitable aperture I62 in the partition I6I. The wave guidemay be formed as an extension of the Walls of the resonator I2fi asshown.

Separated from the gap I21 by a suitable drift space I29 is a gap I30which is defined by opposed truncated conical portions of the discs 2I6and 2H. These discs form portions of the walls of a resonator, e. g., aquarter wave-length coaxial conductor system 2E8, which may have atuning branch 209. A source It!) of alternating potentials is connectedwith the electrode I24, 4

the direct potential of electrode I24 being maintained at a relativelylow potential with respect to the cathode by a source I4I, whereas theconductice portions of the resonator I20, I28 and 208 and the collector3 are maintained at a relatively high positive potential with respect tothe cathode by means of batteries 200 and 20I. The electrode I3 may beconnected, as shown, to the positive terminal of the battery 200. Toeffect the relative potential values hereinbefore specified thepotential of the battery I4I may be relatively low with respect to thecombined potential of batteries 200 and 20I. The tuning branch 209 mayserve to define the short-circuited end or voltage node of the quarterwavelength line 208, another branch 2I0 being provided for connection toany suitable load.

In the operation of the system shown in the drawing, the input wave, thecarrier frequency of which will be designated f1 and which wave may bearsignals in the form of modulations, e. g., amplitude modulations, isresonated in the chamber I20 and produces an electron velocity variationin the electron stream as the latter traverses the associated gap I2I.The frequency of the wave supplied by the local source I40 will bedesignated f2. Electron velocity sorting takes place within the tube I22due to the influence of the relatively low potential of the electrodeI24. With proper selection or adjustment of the battery potentials, acondition is readily secured in which the faster electrons pass throughthe space within the electrodes I24 and I26 whereas the slower electronsare deflected and intercepted by the electrodes I22, I24 and I26, theresult being a charge density variation both in the stream of electronswhich pass beyond the tube I25 and in the current intercepted by theelectrodes. The degree of velocity sorting and resultant groupingproduced is, however, in the present system, under the control of thelocal source I40 by virtue of the variable potential superimposed uponthe direct current biasing potential of the electrode I24 which causesthe resultant, bias to fluctuate. Upon passing the gap I21, thedensity-varied electron stream serves to excite electromagneticoscillations in the resonator I28, which resonator may be tuned torespond to one or both of the frequencies (f1+f2) and (ii-f2), toreinforce these frequencies which, it will be noted, are modulationproducts separated from the frequency of resonator I20 by an amountequal to the value of the frequency of the source I40. Oscillations thuspr'oduced in resonator I28 impress a corresponding velocity variationupon the electron stream as the latter traverses the gap I21. Thevelocity variation of the electron stream entering the drift space I29will contain a component of the local frequency is of source I 40bearing the signal modulations of the original incoming Wave of frequencii. The velocity variation is converted in the drift space I29 intodensity variations at the gap I30 by the grouping effect of shown aprecise or fine tuning arrangement, comprising a branch cavity 202 ofrelatively small dimensions, shown broken open, in which is slidablymounted a piston I53 which may be moved by means of a suitably connectedknob I54. A similar arrangement may be employed in connection with theresonator I20. The branch cavity 202 is preferably of such restricteddimensions compared with the main cavity that the resonant frequency ofthe main cavity is close to and somewhat below the range of freetransmission of the branch cavity so that the latter will not fullysupport a wave of the frequency to be resonated. Then a relatively largemotion of the piston I53 will produce only a relatively small change inthe resonant frequency of the combination. The currents flowing in thebranch cavity will be less than in the main cavity with the result thatany losses associated with the sliding contacts between the piston I53and the walls of the cavity 202 will be minimized.

The resonator I20 is provided with adjustable side cavities 203 and 204for the purpose of enabling the resonator to respond to a relativelywide frequency band. The side cavity 203 is coupled to the main cavityby means of an aperture 205 and is adjustable by means of a piston 206connected to a knob 201. The side cavity 204 is connected to the maincavity and adjustable in a similar manner. The resonator I28 may besimilarly equipped. The aperture 205 is preferably of a suitable size toprovide a loose coupling between the adjacent chambers. The sidecavities may be independently adjusted to resonate at the same frequencyas the associated main cavity or at slightly different frequencies,producing in either case a broad frequency band or relatively flattransmission characteristic in the wellknown manner of coupled resonantcircuits of Whatever form.

In the operation of the system as above described the resonator I20 isassumed broadly tuned to the'frequency ii of the incoming wave andresponsive to the sidebands due to the modulation which the incomingwave may bear. The resonator I28 may be tuned broadly to (f1+f2) or(f1f2) as desired, while the line 208 is tuned to is. It is alsofeasible to operate by tuning the resonator I28 sharply to one of theabove-mentioned frequencies. In that case, the side chambers 203 and204' are not needed and the apertures for coupling the side chambers tothe main cavity should be closed. It is also feasible to operate withthe resonator I 28 tuned so broadly as to include both (f1+f2) and(fi-fz), in which case the line 208 may be energized at twice thefrequency f2 and the length of the line 208 up to the position of thetuning stub should be increased accordingly.

What is claimed is:

A tuning arrangement comprising a substantially closed system ofresonating chambers containing a main chamber and a communicating branchchamber, said branch chamber being of restricted dimensions to determinefor said branch chamber a cut-off frequency close to and somewhat abovethe resonant frequency of the main chamber, whereby a Wave impressedupon said main chamber and substantially resonant therein is attenuatedupon entering said branch chamber, and means for tuning said branchchamber to effect a fine tuning adjustment of the system as a whole.

ARTHUR L. SAMUEL.

